In this era of heightened concern about the relationship between the build up of atmospheric carbon dioxide and climate change, scientists are working to itemize all the ways carbon moves between the atmosphere and the elements of Earth’s surface, including life, water and soil. Forests are of particular interest in large part because many nations now manage the forests within their borders, deciding where and when to harvest trees and when to leave the forest alone. Now those decisions are influenced by the role forests play in the global carbon cycle. Forests’ ability to take in and sequester carbon during photosynthesis has ceased to be something we accept without thought; the biological services they provide have instead become a product with a market value to be traded between nations like radio parts or soybeans. Just as humans have turned to forests for fuel, food, and shelter for hundreds of thousands of years, we now look to them to help us compensate for the atmospheric excesses of our combustion-engine civilization.

Whether or not forests will respond as we hope is unclear. Factors other than carbon dioxide availability influence rates of photosynthesis—factors such as water availability and heat stress. In addition, the carbon cycle of a forest involves more than just carbon dioxide uptake because forests burn, decompose, and respire, re-releasing some of their carbon stash back into the atmosphere. We must consider the contribution of many processes to the overall cycle before we can say what future role forests will play in the global carbon cycle or how much we can rely on them to absorb steadily increasing atmospheric carbon dioxide.

Perhaps nowhere on Earth do questions about the role of forests in the carbon cycle need answers more than in the Amazon Rainforest. The largest expanse of tropical forest on Earth, the Amazon covers just 5% of the Earth’s land surface (neglecting Antarctica), and yet is responsible for 10% of the net primary productivity of the whole terrestrial biosphere. Once an undisturbed, remote, and inaccessible region, the Amazon is experiencing rapid land cover change as a consequence of economic development. In the face of such rapid change, scientists are anxious to understand the carbon cycle of the Amazon.

The Amazon rainforest is a vast area of dense jungle. The vegetation “breathes” carbon dioxide and converts the carbon into biomass—tree-trunks, branches, and leaves. Therefore, the forest acts as a huge reservoir of stored carbon. But other processes, such as respiration and the decay of dead plants, release carbon back into the atmosphere. Scientists are currently studying the forest to learn whether, in the end, the Amazon is a source or a sink of carbon dioxide. (Photograph copyright Digital Vision)

Book-keeping carbonScientists studying the cycle have several sources of information at their disposal. One source is from towers that reach several hundred feet into the top of the forest canopy. Here scientists can measure the flux, or movement, of carbon dioxide into and out of the forest canopy. Another way they retrieve information is by measuring the amount of biomass in the forest. For years, researchers have been monitoring forest plots from the ground, counting trees, measuring them, and sometimes even weighing them to estimate their carbon content. Finally, using mathematical models, scientists sometimes step out from among the trees to take a look at the whole forest. Working backwards from what they know about the global distribution of carbon dioxide in the atmosphere, they try to determine whether the Amazon must be a source of carbon dioxide or a sink, where carbon is stored.

The Amazon Rainforest is a major reservoir of stored carbon. This map shows the amount of carbon absorbed by plants (both on land and in the ocean) per square kilometer, called net primary productivity, in 2002. The Amazon accounts for 10% of the carbon consumed by land vegetation, even though it is only 5% of the land area. (Image by Reto Stöckli, based on data provided by the MODIS Science Team)

The trouble is that the answer to the question of whether the Amazon is a carbon source or sink seems to depend on where you look for the answer. Jeffrey Richey, a biogeochemist at the School of Oceanography at the University of Washington, has been drawn to the puzzle for more than 20 years. “I guess the real question about the carbon cycle in the Amazon is why tropical rainforests aren’t full of monster trees that double in size every year. When we look at the rate of carbon dioxide uptake by forests that is estimated by published results of flux tower measurements, it is a big number—on the order of 3 to 6 tons per hectare each year. That would make the Amazon a huge carbon sink. The problem is, if the trees were taking up that much carbon every year, they would have to be enormous. When we estimate the rate of carbon uptake by looking at the accumulation of carbon in wood and soil, the rates aren’t even close to the flux measurements, perhaps more like 1 to 1.5 tons per hectare per year”, says Richey.

And then there is the global carbon cycle problem. When the scientists who model the carbon cycle on a global scale work backward from the known concentrations of carbon dioxide in the atmosphere, they find that to make the Amazon carbon budget fit into the global carbon balance—with all the known sources and sinks of carbon—the Amazon is either in near equilibrium with respect to carbon exchange—giving off about as much as it takes in—or is possibly even a source of atmospheric carbon dioxide. Could the Amazon be a source of carbon? Imagining all those trees soaking up carbon dioxide day after day makes that idea seem counter-intuitive, if not downright far-fetched.

But whether the Amazon is a small sink, as biomass measurements suggest, or a small source,
as global models suggest, scientists realize that the ecosystem is clearly not the large sink that
the flux tower measurements alone seem to indicate. So where does all that carbon go?